A powerful tornado has struck the outskirts of an eastern Chinese city, killing at least 78 people and destroying buildings, smashing trees and flipping vehicles on their roofs.

The tornado and hailstorm hit a densely populated area of farms and factories near the city of Yancheng in Jiangsu province, abou 500 miles south of Beijing.

Nearly 500 of people were injured, 200 of them critically, the official Xinhua News Agency reported.

Roads were blocked with trees, downed power lines and other debris, while heavy rain and the possibility of further hailstorms and even more tornadoes was complicating rescue efforts, state broadcaster CCTV reported.

The disaster has been declared a national-level emergency, and on a trip to Uzbekistan, Chinese President Xi Jinping ordered central government bodies to provide all necessary assistance.

Tents and other emergency supplies were already being sent from Beijing, CCTV said.

The network showed people carrying the injured to hospitals, cars and trucks lying upside down, street light poles snapped in half, and steel electricity pylons crumpled and lying on their side. Power and telephone communications were knocked out over a broad area.

"I heard the gales and ran upstairs to shut the windows," Xinhua quoted area resident Xie Litian, 62, as saying. "I had hardly reached the top of the stairs when I heard a boom and saw the entire wall with the windows on it torn away."

The roof then collapsed as he raced downstairs, Mr Xie said. After sheltering in a corner for 20 minutes, he emerged to find the neighbourhood transformed into a wasteland. "It was like the end of the world," he said.

Jiangsu is a coastal province north of Shanghai. Yancheng is an ancient city with more than eight million people.

The Jiangsu provincial fire and rescue service provided no word on casualties but said on its microblog that the storm was accompanied by hail. Crews were dispatched to evacuate workers and secure chemicals and other potentially dangerous items at a sprawling solar panel factory in the Yancheng suburb of Funing, it said. No chemical leaks been reported, CCTV said.

Photos posted online showed a wrecked three-story schoolhouse with large trees strewn on its playing field. Its windows had been blown out and its roof and upper floor torn off, along with those of numerous other buildings.

The reports said the tornado hit Funing and Sheyang counties on the city's outskirts the hardest, with winds of up to 78 mph.

Tornados occasionally strike southern China during the summer months, but rarely with the scale of death and damage caused by the one on Thursday.

This year, southern and eastern China have experienced weeks of torrential rain and storms that have caused widespread flooding and dozens of casualties.

The southern part of the country is hit every year during the May-July monsoon season, but this rainy season has been particularly wet. Water levels in some major rivers have exceeded those of 1998, when China was hit by disastrous floods that affected 180 million people, according to state media reports.